Boxwood: Regulating Canopy Density

Fig. 1   Boxwood (Boxus microphylla japonica 'Green Beauty') hedge.

Boxwood is a popular hedge material because it is evergreen with a dense growth  habit.   In my garden hedges of boxwood (Boxus microphylla japonica 'Green Beauty') provide walls around the lawn and divide the yard into different garden rooms.  When other plants die back in the winter, the boxwood hedges remain as strict reminders of the discipline that I have set for my yard. 

When leaves cease to be useful
Healthy leaves of boxwood remain on the plant for two to three years.  I have observed that the leaves that were cut during trimming in the spring of last year are still green and intact (Fig. 2).  Regular trimming maintains the shape of the hedges as it also promotes more branching.  However, as the new growths spread out, the older leaves get covered and deprived of sunlight.  Leaves that do not get enough sunlight become photosynthetically inactive and dependent creating an increased deficit on food supply for the ever increasing foliage.  Eventually, the dependent leaves get less and less ration until the plant decides that they are no longer useful for the plant.  Plants in general, have a very accurate accounting process in this regard.  When a leaf if considered unproductive, the plant will separate it by blocking the passage way for food supply until the leaf is completely dead and detached.  Technically, this process is called abscission.

Fig. 2  Boxwood retains its leaves for more than two years.

When less is more
 
To maintain a healthy boxwood shrub or hedge, the ratio of the active to inactive leaves need to be increased.  As a general rule, all leaves need to get exposed to sunlight. This can be achieved by thinning the branches to allow sunlight to reach into the depths of the canopy.  Thinning also improves air circulation within the hedge thus preventing the growth of diseases. 

Tips for Thinning Boxwood
1.   Cut off branches from within.  From the dense sides of the hedge, cut into the canopy, six to eight inches long - one branch at a time.  Repeat the process throughout until the desired density is achieved.
 
2.  Make way for light and air within.  The process creates open spaces or holes for sunlight and air to penetrate the interior canopy of the plant.  With time, new branches will grow from the inner canopy making the plants healthier and less likely to be infected by fungal diseases.

3.  Change canopy density.  While thinning, keep in mind to maintain the shape of the plant.  Thinning is not intended to change the size or shape of the plant.  Instead, it is the removal of excess branches from a crowded canopy. 

4.  Thin prior to the next growth spurt.  Boxwood put on new growth in the spring and fall.  When the need for thinning arises, it is best to do it before the next growth spurt.  It is easier to spot overlapping branches before the older branches get overgrown by the new ones. 

In my garden, some parts of my hedges are in need of thinning.  One of these days, with my sharp pruners, I will have a date with my boxwood.

It takes years to grow a good hedge of boxwood.  Thinning contributes greatly in keeping them healthy and strong.